1) Field of the Invention
The present invention relates to a document lighting device to be used for a reading apparatus, such as a digital copying machine, an image scanner and a digital laboratory.
2) Description of the Related Art
In recent years, the light emitting diodes (LEDs) have been researched actively. As a result of that, the LEDs are being made brighter and brighter. The LEDs have long service life, high efficiency, high G-resistant property, and they can emit light of one color. As a result, the LEDs are being used in many fields. The LEDs are used, for example, in document lighting devices in image-reading devices such as digital copying machines and image scanners.
FIG. 36 is a schematic diagram of an image-forming apparatus having an image-reading device. The reference numeral 100 represents an image-forming unit and the reference numeral 200 represents an image-reading unit.
The image-forming unit 100 includes a drum-shaped latent-image bearing member 111. A charging roller 112 serving as a charging unit, a developing device 113, a transfer roller 114 and a cleaning device 115 are arranged around the latent-image supporting member 111. A “Corona charger” may be used as the charging unit. Further, an optical scanning device 117 such as an image-reading unit, which carries out an optical scanning process using a laser beam LB upon receipt of document information from an external device, is installed so that “an exposing process through optical writing” is carried out between the charging roller 112 and the developing device 113.
Upon carrying out an image-forming process, the image-bearing member 111, which is photoconductive and photosensitive, is rotated at a constant speed. As a result, the surface of the image-bearing member 111 is evenly charged by the charging roller 112 so that an electrostatic latent image is formed based on exposure through the optical writing by a laser beam LB of the optical scanning device 117. The electrostatic latent image thus formed is a so called “negative-working latent image” with an image portion being exposed. A cassette 118 housing sheets of copy paper P is detachably attached to the main body of the image-forming apparatus 100, and one sheet of the copy paper P on the topmost of the pile of the copy papers is fed by a paper-feeding roll 120, and the leading portion of the fed copy paper P is caught by a pair of resist rollers 119. The resist rollers 119 send the copy paper P to a transferring unit in synchronized timing with the shift of a toner image on the image-supporting member 111 to the transferring position. The copy paper P thus sent is superposed on the toner image at the transferring unit so that the toner image is electro-statically transferred by an action of a transferring roller 114. The copy paper P bearing the transferred toner image is sent to a fixing device 116, and the toner image is fixed in the fixing device 116, and the resulting copy paper P is discharged onto a tray 123 by a pair of paper-discharging rollers 122 through a transferring path 121. After the toner image has been transferred, the surface of the image-bearing member 111 is cleaned by a cleaning device 115 so that residual toner, paper powder and the like are removed. The latent-image bearing member 111, which is a photoconductive photosensitive member, forms the electrostatic latent image through uniform charging and optical scanning processes, and the electrostatic latent image thus formed is visualized as a toner image.
In the image-reading unit 200, a document 202 is placed on a contact glass plate 201, and a lighting unit (not shown) which is installed on a first moving member 203 placed below the contact glass plate 201, illuminates the document 202. Light rays, reflected from the document 202, are further reflected by a first mirror 203a of the first moving member 203, and then reflected by a first mirror 204a and a second mirror 204b of a second moving member 204 to be directed to a reduction image-forming lens 205 so that an image is formed on a line sensor 206.
Upon reading a document in the length direction, the first moving member 203 is shifted rightward in the figure at a speed V, and simultaneously, the second moving member 204 is shifted rightward at a speed ½·V so that the reading process is carried out over the entire document.
Normally, a document-lighting device to be used for the image-reading device needs to have virtually the same length as the document width to illuminate the document; therefore, with respect to the application method for LEDs as the document-lighting device, a number of LED elements are arranged, and used as an array format.
Although the current LEDs have the superior characteristics, absolute brightness of each of the elements is not sufficient so that they are not suited as a lighting device for an image-reading device. Because of this drawback of the LEDs, their use is restricted to low-speed reading apparatuses and compactness-serious apparatuses. On the other hand, cold cathode-ray fluorescent lamps are mainly used in high-speed reading apparatuses and large-size apparatuses.
One approach to solve the problem with the LEDs is to use a number of LEDs and constitute an LED array to thereby increasing total quantity of light. However, since light of the LED array diffuses widely, this approach is not so effective. Moreover, since the LED array consumes great power, this approach opposes the current energy-saving demands.
FIGS. 37 and 38 are cross-sectional views of a lighting device in which a rod-shaped light source is used. The reference numeral 1 represents the rod-shaped light source such as a cold cathode-ray fluorescent lamp, 2 and 2′ represent mirror-face members, each having a partially cylinder-shaped concave-face reflecting unit serving as an optical element, 3 represents an illumination face such as a document face, and 4 indicates a luminance distribution curve in the sub-scanning direction on the illumination face. For convenience of explanation, the mirror-face member is indicated by only the reflection face. The same is true for the other figures.
Total quantity of light output from the rod-shaped light source is increased in the following manner. That is, light rays output from the rod-shaped light source 1 are reflected by the mirror-face member 2 having the partial cylinder shape so that the light rays are collected on the document face 3. Here, the partially cylinder shape, which is also referred to as a cylindrical shape, refers to a shape that has a cross section corresponding to one portion of a quadratic curve, such as a circle, an ellipse, a parabolic curve and a hyperbolic curve, or a shape close to one of these shapes, and is designed so that the length of the light source in the length direction of the rod-shaped light source is set to virtually the same length as the length of the illumination face.
FIG. 39 is a schematic diagram that depicts a positional relationship of light-receiving elements in a digital copying machine and an image scanner. The reference numeral 5 represents an image-forming lens, 6 represents a light-receiving element, and 7 represents a single light-receiving unit.
In the digital copying machine and the image scanner, as shown in the figure, light rays, reflected from a document, are received by the single light-receiving unit 7 of the light-receiving element 6 through the image-forming lens 5. In the light-receiving element 6 such as a CCD sensor, the width of the single light-receiving unit 7 is normally as narrow as 0.05 millimeter to 0.1 millimeter. In other words, in the case of the equal-magnification image-forming process, only the area having the corresponding narrow width is read on the document face. Therefore, as shown by the example of FIG. 38, when light rays from the light source are sharply converged, the position of the luminance distribution curve 4 tends to deviate due to deviations in illumination position caused by deviations and the like in a mirror angle to greatly vary the quantity of light to reach the single light-receiving unit 7, with the result that an image to be formed is greatly affected.
The figure depicts an example in which an equal-magnification sensor is used, and, for example, even when a 1/10 reduced image is formed by using a reduction optical system, the width of the illumination area on the document side to be image-formed on the single light-receiving unit 7 becomes only 1 millimeter at most, with the result that the same problems are raised.
FIGS. 40A, 40B, 40C, and 40D are for explaining how an illumination distribution curve changes with the image-reading position. FIGS. 40A and 40 B depict a case in which the width of an illumination distribution curve is comparatively narrow, and FIGS. 40C and 40B depict a case in which the width of an illumination distribution curve is comparatively wide. Here, the term, “comparatively”, refers to a width in comparison with the width of the image-reading area.
FIGS. 40A and 40C depict normal states.
In digital copying machines, the width of the light-receiving unit is as narrow as 0.1 millimeter. Therefore, as shown in FIG. 40B, when the center position of the illumination distribution curve 4 deviates from the reading portion, the luminance of the reading area drops greatly. Thus, in the digital copying machine and the image scanner, there have been demands for a document lighting device which has a wide luminance distribution curve 4 in the sub-scanning direction as shown in FIGS. 40C and 40D, and is less susceptible to luminance difference in the reading area even when the center position of illumination deviates from the reading portion. For this purpose, it is preferable to form a portion with little luminance irregularity that has a width greater than the width (for example, approximately 1 millimeter for one side) consisting of a width required for the reading process (approximately, the maximum 1 millimeter in the example) and a fluctuation width (for example, approximately 1 millimeter for one side) due to mechanical errors and the like added thereto, near the maximum value in luminance distribution, that is, a flat portion in luminance.
To satisfy these conditions, with respect to the application of a rod-shaped light-source-use reflecting mirror, an arrangement has been proposed in which, as shown in FIG. 37, a document face is illuminated with a wide width without converging too much light although the efficiency is low, or an appropriate luminance distribution is formed by combining a plurality of planes (for example, see Japanese Patent Application Laid-Open No. 6-22087 (page 3, FIG. 1)). However, since the premise of this arrangement is to use a rod-shaped light source, it is difficult to apply this method to LEDs having a size much smaller than this light source. It is mainly because there is a difference in size ratios between the width of the illumination face and the light-emitting unit of the light source. The width in the sub-scanning direction of the illumination face is about several millimeters; however, the size of the light-emitting unit of the rod-shaped light source is greater than this size, while the size of the light-emitting unit of the LED is smaller than the width of the illumination face. This difference causes differences in the distance between the light source and the illumination face and in the size of reflecting mirror.
The application of the same structure as that of FIG. 37 in an attempt to use an LED array fails to achieve the main objective for compensating for insufficient brightness of the LEDs since the light utilization efficiency is very low.
Another arrangement for utilizing the LEDs as an illumination light source has been proposed (for example, see Japanese Patent Application Laid-Open No. 2002-93227 (page 4, FIG. 1)). However, the arrangement suggested in Japanese Patent Application Laid-Open No. 2002-93227 describes nothing about the luminance distribution in the illumination area.
The applicant of the present invention has proposed an arrangement that provides a luminance distribution similar to luminance distributions shown in FIGS. 40C and 40D (Japanese Patent Application No. 2003-140927). In this arrangement, a light-incidence face is placed near the light-ray-releasing face of a point light source as an optical element, and a light-directing member is prepared with its light-releasing face facing the reading area. With this arrangement, the target luminance distribution is exemplarily obtained; however, a reflection plate is required in addition to the light-directing member so that the arrangement becomes more complex, resulting in the subsequent high costs.